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Reaction Rate and Equilibrium. Reaction Rate. Not all reactions occur at the same speed. Some reactions are very slow while others are fast. Reaction Rate.
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Reaction Rate • Not all reactions occur at the same speed. • Some reactions are very slow while others are fast.
Reaction Rate • The reaction rateof a chemical reaction is a measurement of the increase in the concentration of a product or the decrease in the concentration of a reactant as the reaction proceeds over time.
N2 + 3H2 2NH3 • The units generally used to express reaction rate are mol/L . sec. • What does it mean if the rate of the reaction above was given as 4.5 x 10-2 mol N2/L . sec?
C(s) + O2(g) → CO2(g) • Occurs slowly at room T° and normal [O2]. • Can speed up the reaction by increasing the T° or increasing [O2]. • We can explain these changes in reaction rate using the “collision theory”.
Collision Theory • All substances are comprised of millions of tiny particles in constant motion. These particles are colliding with each other constantly in any substance. • All collisions between particles do not result in a reaction. • There are two factors that determine whether or not a reaction will occur between two particles that are colliding.
Collision Theory • Substances most come into contact, (collide) with enough energy. Activation Energy has been supplied by the force of the collision.
Activation Energy • The activation energy is the amount of energy that must be available in order for a reaction to occur.
The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. • The blue flame will sustain itself after the sparks are extinguished because the continued combustion of the flame is now providing the necessary energy through an exothermic reaction.
Collision Theory • Substances most come into contact, (collide) in the correct orientation (facing the correct way).
Collision Theory • The collision theory states that reacting substances most come into contact, (collide) with enough activation energy, and in the correct orientation (facing the correct way), so that their electron shells can rearrange to form the products of the reaction. • Therefore any factor which changes the frequency (how often), or energy of the collisions will change the rate of the reaction.
Five Factors AffectingReaction Rate • nature of the reacting substances • concentration • surface area • temperature • catalysts
Nature of the reacting substances • The type, strength, and number of chemical bonds or attractions between atoms differ from one substance to another. • These differences determine the energy and orientation of the reacting particles that is necessary to create an effective collision resulting in a reaction.
TNT vs Gunpowder • Explosive materials which react very violently are known as high explosives. In contrast, there are some materials that react more slowly. These are known as low explosives. They release a large amount of energy, but due to the relatively slow rate of reaction.
TNT vs Gunpowder TNT C6H2(NO2)3CH3 is a high explosive. Gunpowder is a mixture of potassium nitrate (KNO3), sulphur (S8) and charcoal (C). It is a high explosive.
As the concentration of the reactants increases, the reaction rate increases. Why?
Concentration • Concentration of the reactant refers to the number of reactant particles within a given volume. • If the concentration of the reactants increases there will be a greater number of collisions. • The greater the number of total collisions, the greater the number of “effective” collisions (collisions that will form product) and the greater the rate.
Concentration and Reaction Rate Steel wool burning in air Steel wool burning in pure oxygen A (21% oxygen) (100% oxygen)
As the surface area of the reactants increases, the reaction rate increases. Why?
Surface Area • Increasing the surface area of the reactants results in a higher number of reaction sites. • Reaction sites - specific sites on molecules at which reactions occur. • Increasing the number of reaction sites increases the number of total collisions. • The greater the number of total collisions, the greater the number of “effective” collisions (collisions that will form product) and the greater the rate.
Reaction Rate and Surface Area – Lycopodium Powder Exit slideshow and play using Windows Media Player. There is a delay if we try to show this file within the slide show.
As the temperature of the reactants increases, the reaction rate increases. Why?
Temperature • Increasing the temperature increases the kinetic energy of the particles. • This results in more frequent collisions and more energetic collisions. • Therefore not only are there more collisions but also a greater percentage of the collisions have the needed activation energy.
Both light sticks have been activated however the one on the left was placed in ice water and the one on the right in boiling water.
Catalysts • Catalysts lower the activation energy by providing an alternate pathway by which the reaction can occur at a lower energy. • This results in a greater percentage of the collisions having the necessary energy to be effective resulting in an increase in reaction rate. • Catalysts are remain unchanged at the end of a reaction.
Left: Partially caramelized cube sugar, Right: burning cube sugar with ash as catalyst
Enzymes • Enzymesact as catalysts that lower the activation energy of a chemical reaction within a living organism. • Enzymes carry out their function of lowering activation energy by temporarily combining with the chemicals involved in the reaction. These chemicals that the enzyme combines with are called the substrate. • When the enzyme and substrate combine, the substrate is changed to a different chemical called the product. The enzyme is not consumed or altered by the reaction.
Enzymes • Enzymes are specific for their substrate: A particular substrate molecule will combine temporarily with one enzyme type, and the active siteof a particular enzyme will fit only one kind of substrate. For example, the enzyme sucrase will attach only to the substrate sucrose.
Homework • Worksheet: Reaction Rate
Reversible Reactions N2 + 3H2→ 2NH3 • Reactions can normally be reversed. 2NH3→ N2 + 3H2 • Reversible reactions are often indicated by a double arrow (↔). N2 + 3H2↔ 2NH3 • This shows the forward and reverse reaction.
For a reversible reactionsuch as A + B ↔ C + D there are actually two reactions Forward reaction: A + B → C + D Backward reaction: C + D → A + B
A + B ↔ C + D Forward reaction: A + B → C + D A and B are used up C and D are formed Backward reaction C + D → A + B C and D are used up A and B are formed
At beginning of the reaction A + B ↔ C + D There is only A and B in the reaction container (no C and D formed yet) Forward reaction is very fast No backward reaction occurs yet
A little later A + B ↔ C + D Forward reaction is still fast (since the container still has mainly A and B) Some C and D have been formed The backward reaction starts Backward reaction is very slow (since there is only a small amount of C and D)
Still some time later A + B ↔ C + D More A and B have been used up The forward reaction slows down More C and D have been formed The backward reaction speed up
Eventually… A + B ↔ C + D the point is reached where the speed (rate) of the two reactions become equal. The system is then said to be in EQUILIBRIUM
Chemical Equilibrium At equilibrium • The rate of the forward reaction becomes equal to the rate of the reverse reaction. • How could this graph be adjusted and still show equilibrium?
Chemical Equilibrium At equilibrium • The forward and reverse reactions continue at equal rates in both directions. • For this reason we often refer to a “dynamic equilibrium”
